Synergy And Transition Mechanism Of Nitrogen Use Efficiency In Various Genotype Rice Under Organic Farming

In rencent years, organic rice, being a healthy and safe organic food, has developed rapidly and had huge market potential in China. However, the mechanism of nitrogen absorption in organic rice is still currentlly unknown. To investigate this issue, in 2013 and 2014, this study was conducted in an organic farm in Gaoyou city, Jiangsu province. Thirty-two selected conventional representative Japonica rice varieties, with similar growth stage and various nitrogen use efficiency (NUE) under conventional farming were field-grown. Four treatments of organic farming (OF), including CK, three (low, medium and high) levels of organic fertilizer (designated LO, MO and HO respectively), and conventional farming (CF,270 kg-hm"2) were conducted from all the growth period of rice. Through this study, special germplasm resources of organic rice with high NUE will be screened. The NUE mechanism of organic rice will also provide a scientific basis for organic rice cultivation, selection of species and nutrient management. The main results were as follows:1. With the increase levels of organic fertilizer, the differences between yields of various rice genotypes have a decreasing trend. The corresponding treatment of organic fertilizer level, under which achieving the maximum productivity for 32 rice varieties, is also different. Most genotypes maintained their maximum productivity under treatments of middle and high levels of organic fertilizer. Under organic farming, the maximum productivity ranged from 5.31 to 8.45 t·hm-2, the coefficient of variation between varieties was 10.96%. According to the highest productivity under OF, all the varieties could be classified as three levels:the top level (HY, the highest grain yield> 6.75 t·hm-2), the middle level (MY.6.0 t·hm-2≤ highest grain yield≤6.75 t·hm-2) and the low level (LY, the highest grain yield<6.0 t·hm-2). Nitrogen use efficiency of organic rice ranged from 12.12% to 35.36%. With the increase of organic fertilizer application, nitrogen use efficiency firstly increased and then decreased, maintaining the highest NUE at treatment of middle-level organic fertilizer. Based on the highest yield and the corresponding NUE, the rice varieties were classified under organic cultivation.2. Under conventional farming, NUE of various rice genotypes ranged from 31.44% to 45.83%. After comparison the difference of NUE between OF and CF, the 32 genotypes of rice could be classified as four types:with high NUE under both organic and conventional farming (synergistically high NUE type, A-type); with high NUE under OF and low NUE under CF (high-low NUE transition type, B-type); with low NUE under OF and high NUE under CF (low-high NUE transition type, C-type); with low NUE under both organic and conventional farming (synergistically low NUE type, D-type).3. Among four types of representative rice, Zhendao 15, Nanjing 46, Huaidao 5 and Suxiangjing 1 were selected as representative genotypes as types A, B, C and D respectively. From booting to maturity stage, physiological activity of flag leaves of those four rice genotypes under organic and conventional farming. Under OF, high value of SPAD and nitrogen content, decreasing slowly during the growth period, were observed in B-type genotypes rice. Whereas, under CF, those indexes decreased slowly in C-type. The magnitude of decline in nitrogen content of C-type and D-type was bigger under OF than that under CF. As for GS activity of flag leaf, the decline magnitudes of A-type and B-type were smaller under OF than those under CF. Whereas, C-type and D-type had the opposite trends. The influences of cultivation patterns on the GPT, GOT activities in flag leaves were similar as GS activities. So it demonstrated that organic farming could slow the decline rate of key nitrogen metabolism enzyme activity in A-type and B-type, thus maintaining relatively high capacity of nitrogen uptake and utilization during the late growth period of rice. Moreover, organic farming could increase the content of SOD and decrease the content of MDA in A-type and B-type and delay leaf senescence. On the contrary, OF speeded up leaf senescence of C-type and D-type. Compared with the other genotypes, organic farming significantly improved the appearance quality, the rice palatability, RVA characteristics and the content of P, K, Ca, Mg, Fe, Mn, Zn, B, etc in B-type rice.